1. Field of the Invention
The present invention is in the field of noise reduction and is specifically directed, in its preferred embodiment, to an apparatus and method for reducing fuselage interior noise in a propeller driven aircraft. However, the invention is also usable in noise reduction in other non-aeronautical environments.
2. Description of the Prior Art
There has been a substantial amount of work done in connection with propeller driven aircraft and, in particular, with the proposed advanced turbo-propeller (ATP) aircraft due to their inherent high fuel efficiency. A drawback of such ATP aircraft resides in the high levels of sound within the aircraft's cabin due to the higher tip velocities of their rotating propellers and the large amplitude vibrations induced in the wings by engine vibration harmonics and propeller wakes. Hence, the prior art has been concerned with reduced noise transmitted through the fuselage and into the cabin interiors of such aircraft.
Investigations as reported in "Characteristics of Propeller Noise on an Aircraft Fuselage Related to Interior Noise Transmission," by J. S. Mixson et al., AAIA 79-0646, 1979, and in "Development and Validation of Preliminary Analytical Models for Aircraft Interior Noise Prediction," by L. D. Pope et al., J. Sound Vib., 82(4), 1982, conclude that aircraft noise could best be reduced by the use of panels having characteristics of mass, stiffness and damping, which minimize the transmission of sound therethrough, when installed in an aircraft fuselage. Such panels are passive in nature and only work well for high frequencies. Since propeller noise will vary in frequency in accordance with the aircraft operating conditions, such panels do not provide for full-time noise reduction in the low frequency region where the major noise of ATP's resides. Another approach to aircraft noise reduction is the use of absorbent damping material placed in the inside of the aircraft fuselage for damping out the acoustic field. In general, propeller noise is of a very low frequency in the order of 100 to 250 Hz, having a corresponding long wavelength. This absorbent damping is ineffective against such long wavelengths, working well only at high frequencies. The disadvantages associated with these solutions are thus the weight added to the aircraft, plus the lack of low frequency attenuation.
Another approach to the problem of noise reduction in fuselage interiors of aircraft has been to use tuned mechanical vibrators. In such installations, small mass springs are attached to the fuselage surface at various interior locations. These springs have resonant frequencies which cause them to vibrate in response to being subjected to sound at a particular frequency. Such systems are not active and only work at one single frequency. Moreover, such systems do not provide for good global attenuation unless many springs are employed, thus adding weight to the aircraft; such springs are totally useless when the sound in the aircraft cabin operates at frequencies other than their one design frequency, and also tend to be fragile.
The publication entitled "Propeller Signatures and their Use" by J. F. Johnson et al., AIAA, 80-1035, 1980, discusses the synchrophasing of propeller blades to reduce aircraft noise in propeller driven aircraft. Synchrophasing involves the adjustment of the relative rotational phase of different propellers on the aircraft to effect noise reduction. Synchrophasing operates to effect a noise reduction over a reasonable volume of the order of 8 dB in sound pressure level. Apparently, synchrophasing provides significant noise rejection rather than a redistribution of the acoustic energy within the aircraft cabin, implying that active methods which affect fuselage vibration will be successful in providing global attenuation inside the aircraft cabin.
An active noise cancellation system is described for an ATP aircraft in a NASA report number CR172386 entitled "Active Attenuation of Propeller Blade Passage Noise," by Zalas et al. This active noise cancellation system for the interior of an aircraft, includes an input sensor for providing a signal indicative of the rotational velocity of the aircraft's propeller or the frequency content of the propeller noise. The input sensor may include a synchrophasor signal from the aircraft engines, an accelerometer coupled to the fuselage wall of the aircraft, an interior microphone or an exterior microphone. The input signal is applied to a controller in the form of a computer for executing a modified Widrow-Hoff least means squared (LMS) algorithm for adjusting the amplitude and phase of the signal applied to a loudspeaker or loudspeakers disposed within the aircraft cabin, to minimize the sound level at a particular location(s) with the aircraft cabin. The controller output is amplified before being applied to the loudspeaker(s), which output a sound of appropriate phase and amplitude to cancel the objectionable noise. An error sensor is disposed within the aircraft fuselage to apply an error signal proportional to the residual noise, to the controller, which adjusts its output signal as to phase and amplitude to achieve a minimum noise level within the aircraft cabin. To achieve optimum noise cancellation, the loud speaker should be relatively small with respect to the wave length of the noise and of approximately the same size as the noise source, and further should be disposed as close to the noise source as possible. The acknowledged drawback of this system was that noise cancellation did not occur at positions within the interior volume of the aircraft other than where the error sensor was placed. In particular, noise cancellation occurred, wherein a single cancellation speaker was used, of less than a quarter wave length around the error sensor. This publication recognizes the limitations of this method to achieve global attenuation throughout the volume of the aircraft cabin and suggests that further studies be conducted to determine the mechanism and characteristic of sound transmission into the interior of an aircraft's fuselage to identify the points at which sound is introduced therein and to employ an array of the speakers disposed within the aircraft cabin to potentially achieve global noise reduction.